The amount of packet-based data traffic in today's telecommunication networks is increasing steeply, and in many networks has already overtaken circuit-switched traffic. In order to answer the exploding demand for network capacity, wavelength division multiplexing (WDM) is being introduced in core networks, in metropolitan area networks, and in access networks.
In most current systems, Internet Protocol (IP) packet-based data is typically carried over WDM, with synchronous digital hierarchy (SDH) or asynchronous transfer mode (ATM) as the intermediate layers. However, avoiding these intermediate layers by carrying IP directly over WDM yields more efficient and cost-effective networks, and is generally considered to be the preferred approach in data-centric networks. IP-over-WDM is supported by an optical multi-protocol label switching protocol (MPLS) formerly known as MPλS protocol, an optical multi-protocol wavelength switching protocol, and now part of Generalized Multi-Protocol Label Switching (GMPLS) particularly with reference to the Lambda-Switched Capable (LSC) level, where label-switched paths (LSP-s) are being associated with optical channels. Optical packets (or bursts of packets) are switched to other wavelengths (label swapping or switching) at transmitters in network edge nodes and (optionally) in the intermediate network core nodes. Wavelength routers in the core nodes guide the packets along the appropriate network paths.
In addition to optically labeling a packet (or bursts of packets) by assigning a particular wavelength to it, a next-level label can be attached by using a particular modulation scheme. The latter may be accomplished by modulating the label information orthogonally to the packet data (i.e., modulating the phase of the optical carrier (FSK or DPSK) while the packet data is modulated on the intensity of the optical carrier). Having two optical labels is in agreement with label stacking as foreseen in MPLS, and is in accordance with switching or routing part of the streams group-wise. Henceforth, frequency shift keying (FSK) is considered as the second optical label level. To interpret the contents of the FSK label at label-controlled nodes, the FSK label needs to be separated from the amplitude shift keyed (ASK), or intensity modulated (IM) in general, payload at the incoming port. Generally, the use of a frequency discriminator, such as a narrow band optical filter, offers this capability and functions optimally for continuous bit-rate signals. However, in packet-switched networks, the receivers must function with burst-mode traffic. In such systems, correct data recovery of the optical FSK label will be hampered by residual ASK (or IM) modulation from the payload. Depending on the ratio between the ASK (or IM) and FSK bit-rates and the payload contents, the impact of this residual ASK (or IM) effect can vary from negligible to severe.
Demodulation of the FSK label by frequency discrimination acts as a moving average over the payload signal multiplied by the FSK label information (“0” or “1”). Variation of the payload contents over time will appear on the received “1” level of the FSK label. In the case of long run-lengths in the payload and/or small bit-rate ratios between the ASK (or IM) and FSK data streams, dynamic determination of the right threshold level for label data recovery is extremely difficult.